This invention relates to blind stud inserts and particularly to the manufacture and assembly of an insert body and an anchor stud.
The invention relates to a method of manufacturing a blind stud insert from metal, which insert can be inserted through a workpiece and radially expanded to secure it to the workpiece. In this context, “blind” means that the insert can be installed by access to one side only of the workpiece.
It is known to provide a threaded insert body which is screwed then staked onto an anchor stud. However, the production and assembly of this type of insert involves many steps, including heat treatment of the anchor stud, roll tapping, washing and spinning of the body onto the anchor stud, and staking of the body onto the anchor stud. The heat treatment of the anchor stud must render the threaded shank sufficiently hard to enable it to withstand the force of the staking stage.
The rate of assembly of the above inserts is limited by the time-consuming processes of roll-tapping and of spinning of the body onto the anchor stud, with the time taken for this latter process increasing with the length of the anchor stud. A problem may also occur with cross threading between the body and the anchor stud threads, thus further increasing assembly time.
It is also known to provide an unthreaded body, which is staked directly onto the anchor stud. Although this embodiment does not suffer from the disadvantage of time-consuming spin-on, it has been found that stud retention is poor as the grip between the body and the threaded shank of the anchor stud is insufficient.
Another prior art insert, as disclosed in EP1598567 (Böllhoff) involves crimping an unthreaded body onto an anchor stud which is provided with longitudinal grooves superimposed over a helical thread form. This design compromises the torsional strength of the threaded shank of the anchor stud in the grooved region. The effective area of interlocking between the grooved thread crests of the anchor stud and the displaced material of the body is relatively small and consequently problems of adhesion between the body and anchor stud could still occur with this embodiment. When a high torque is applied to the insert, it is possible that the body and anchor stud can become separated. High torque could be applied, for example, on installation of the fastener, or when cross-threading occurs between the nut being assembled and the installed anchor stud to form the final joint and as a result the stud unscrews from the body. It is also possible that the body and anchor stud can become separated as a result of axial forces applied to the workpiece into which the insert has been installed.
The final joint may need to be undone and re-assembled in-service. Removal and re-tightening of the nut, especially if corroded, will apply torque loads to the stud in both directions of rotation and may also cause separation of the stud with the body.
After installation of an insert, it may become necessary to remove it from the workpiece into which it has been installed, for example for disassembly of the workpiece for end-of-life recycling. Inserts must therefore also be able to withstand the axial force necessary to force the insert out of a workpiece in either direction, without the anchor stud and body becoming separated.
It is an aim of the present invention to overcome or at least mitigate the aforementioned problems.
Accordingly the present invention provides, in one aspect, an insert for blind installation in a workpiece comprising a body and an anchor stud; the anchor stud having an elongate portion and a head located at or near one end of the elongate portion and extending radially therefrom; the anchor stud being insertable into a through bore extending throughout the interior of the body, such that when inserted, a first section of the elongate portion is enveloped by a reduced diameter section of the body, and a second section of the elongate portion, remote from the head, extends beyond an end of the through bore; wherein the first section of the elongate portion is provided with at least two areas of opposing angled knurling.
An advantage of the present invention is that during the crimping stage of the assembly of the insert, material is caused to enter the interstices of the anchor stud knurling, thus firmly holding the body onto the anchor stud. Therefore the insert can withstand the application of torque without allowing the anchor stud to become separated form the body. The provision of two areas of opposing angled knurling enables the insert to cope with such torque in either direction. The application of torque has no net effect in terms of generating axial movement in either direction even when the frictional force has been overcome. If two areas of knurling were provided with like angling, the effect would be similar to that of a steep helix or helical thread, i.e. the application of torque will produce a tendency for axial movement in one direction once the frictional forces have been overcome.
Preferably the insert is provided with one or more annular grooves in the first section of the anchor stud, i.e. the section which is enveloped by the body. More preferably, an annular groove is provided between the two knurled areas. An advantage of providing annular grooves is that during the crimping stage of the assembly of the insert, material from the body is also caused to flow into the annular grooves, thus acting to lock the body onto the head and resist axial forces applied to the insert, for example if it becomes necessary to remove the insert after installation.
Preferably, the anchor stud is made of dual-phase steel which has been work-hardened during cold forming and thread rolling operations, thus decreasing the cost and complexity of manufacture by eliminating the requirement for hardening and tempering of the stud. The use of dual-phase steel also eliminates the potential for hydrogen embrittlement caused by the hardening, tempering, and surface finishing processes, whilst ensuring that the anchor stud is sufficiently hard to cope with the forces encountered on crimping the body onto the anchor stud.
In an alternative embodiment, the body 4 could be of a different external cross-sectional form, such as square or hexagonal, to be inserted into a correspondingly shaped workpiece aperture. A body having a non-circular external cross-section would provide an advantage in terms of increased resistance to rotation of the body within the workpiece on application of torque to the stud.
The insert body could also be provided with exterior knurling 32, or underhead barbs 26a on the body flange, which embed in the workpiece to increase resistance to rotation on application of torque.
The invention provides, in a second aspect, a method of manufacture of an insert which method comprises steps of: forming an anchor stud and a body, wherein the anchor stud is formed with an elongate portion and a head located at or near one end of the elongate portion and extending radially therefrom; inserting the anchor stud into an unthreaded through bore extending throughout the body, such that areas of opposing angled knurls provided on a first section of the elongate portion are enveloped by a reduced diameter section of the body, and a second section of the elongate portion, remote from the head, extends beyond an end of the through bore; and crimping the reduced diameter section of the body enveloping the knurled areas of the anchor stud such that material of the body is forced into the interstices of the knurls.
An advantage of the method of manufacture of an insert according to the present invention is that subsequent to the forming of the anchor stud and body, the assembly of these two components to form the insert involves only two stages, i.e. inserting the anchor stud into the body, and crimping the body onto the anchor stud. Thus assembly of the insert is simplified as compared with the prior art methods which involve screwing the anchor stud into a threaded body, and the potential mode of failure by unscrewing of the two components is eliminated. Furthermore, the resulting adhesion of the parts is sufficient to cope with subsequent application of torque to the insert, in either direction. Therefore the likelihood of the insert body becoming separated from the anchor stud on application of high torque applied to the insert is significantly reduced.
A further advantage of crimping the body onto the anchor stud is that any corrosion-resistant plating or coating which has been applied to the body or anchor stud prior to assembly is not significantly affected.
The invention provides, in a further aspect, an insert for blind installation in a workpiece comprising a body and an anchor stud; the anchor stud having an elongate portion and a head located at or near one end of the elongate portion and extending radially therefrom; the anchor stud being insertable into a through bore extending throughout the interior of the body, such that the elongate portion has a first section which is enveloped by a reduced diameter section of the body, and a second section, remote from the head, which extends beyond an end of the through bore; wherein the elongate portion is provided with threading extending from part of the first section to at least part of the second section, whereby the threading extending into the second section acts as a first area of knurling, and wherein the first portion is provided with at least a second area of knurling at an opposing angle to the pitch of the threading.
An advantage of providing a threaded anchor stud wherein the threading extends into the first section of the stud, i.e. into the region onto which the body is subsequently crimped, is that the threading may act as an area of knurling, wherein body material will be forced into the threading on crimping as in the first embodiment.